1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic or electrostatic recording apparatus, and more particularly, it relates to an image forming apparatus wherein a recording medium bearing means for transferring a toner image formed on an image bearing member onto a recording medium has a specific feature and which is applicable to a multi-color image forming apparatus such as a multi-color or full-color electrophotographic copying machine, printer or the like.
2. Related Background Art
A color image forming apparatus in which toner images are successively formed by toner having a plurality of colors on an image bearing member constituted by a rotary cylinder, endless belt or the like while rotating the image bearing member continuously, and these toner images are successively transferred onto a recording medium carried by an recording medium bearing means at a recording position where the image bearing member and the recording medium bearing means are opposed, thereby obtaining a color image has been already known.
FIG. 8 shows an exemplary conventional color electrophotographic copying machine.
In FIG. 8, the color electrophotographic copying machine is so designed as to form electrostatic latent images on an electrophotographic photosensitive drum 100 acting as an image bearing member by means of a charging means 200 and an exposure means 300, to change the latent image into color toner images by means of a horizontally shiftable developing means 400 including a plurality of developing devices 400Y, 400M, 400C and 400BK including, for example, yellow toner, magenta toner, cyan toner and black toner, respectively, and to transfer these color images in the superimposing fashion onto a sheet-shaped transfer material P such as a paper electrostatically attached to a transfer material bearing device 1 acting as a drum-shaped recording medium bearing means disposed in the vicinity of the photosensitive drum 100, thereby obtaining a color image.
As briefly shown in FIG. 7, the transfer material bearing drum is constituted by integrally connecting a pair of coaxial ring members 1a and 1b by means of a connector member 1c and by covering these elements 1a, 1b, 1c with a flexible dielectric sheet 3.
Transfer material gripping means 4 are provided on the connector member 1c of the transfer drum 1 so constructed, so that a leading edge of the transfer material P fed from a cassette (not shown) in the timed relation to the toner images on the photosensitive drum 100 is gripped by the gripping means 4. The transfer drum 1 gripping the transfer material P approaches the transfer material to the photosensitive drum 100 and releases the transfer material P after the color toner images have been transferred onto the transfer material to form the color image. The transfer material P separated from the transfer drum 1 by means of a separating means 500 including a separating charger, separating pawl and the like is fed to a fixing means 700 by a conveying means 600, where the toner image is fixed on the transfer material.
As shown in FIG. 7, the operation of the transfer drum 1 is affected by a gear 14 rotated integrally with the ring member 1a of the transfer drum 1 and meshed with a gear 13 rotated integrally with the photosensitive drum 100, and the rotating force is transmitted from the ring member 1a to the ring member 1b through the connector member 1c.
In general, the transfer drum 1 is so designed that it can be manually moved toward and away from the photosensitive drum 100 so that, when they are separated, the gear 14 is disengaged from the gear 13 to manually rotate the transfer drum 1 freely, thereby permitting easier unjamming treatment when the transfer material is jammed.
Heretofore, a plurality of transfer materials were rarely carried by the transfer drum 1 having the above-mentioned arrangement simultaneously. Thus, the connector member 1c had a dimension sufficient to keep the rigidity of the drum and, the rotational driving force could be transmitted from the ring member 1a to the ring member 1b only through the connector member 1c.
However, in the transfer drum 1 having such arrangement, if it is desired that the diameter of the drum is reduced or a plurality of transfer sheets are carried by the transfer drum, the connector member 1c sometimes could not provide the sufficient rigidity of the drum.
Further, as shown in FIG. 7, when the transfer drum 1 is driven, the connector member 1c is always subjected to the rotational driving force, the inertia moment of the ring member 1b and a component (F) of force in a rotational direction of a load such as a load generated due to the sliding action in bearing portions, thus twisting the connector member 1c.
If the connector member 1c cannot provide the sufficient rigidity of the transfer drum as mentioned above, such twist or torsion of the connector member will be greater.
Accordingly, if a plurality of transfer materials are subjected to the image transfer treatment continuously, the image formed on the later transfer material will be more distorted or extended at the ring member 1b, in comparison with the image formed on the first transfer material. Further, the twist or torsion of the connector member 1c affects a bad influence upon the dielectric sheet or transfer material bearing sheet 3 so that the blank portion and/or color deviation occur in the transferred image and that the service life of the transfer material bearing sheet is decreased.
Thus, it was practice that the connector member 1c was made of material having the greater strength and was designed to have the wider area. However, this increases the production cost of the transfer drum and limits the size and/or number of the transfer materials to be born by the drum.